Fair Value Measurements - Fair Value Hierarchy for Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | $ 2,054 | $ 1,998 |
Total assets | 3,143 | 2,898 |
U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 265 | 225 |
State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 76 | 148 |
Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 1,581 | 1,503 |
Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 73 | 60 |
Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 59 | 62 |
Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 287 | 301 |
Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 690 | 498 |
Other long-term investment | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investment | 112 | 101 |
Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 708 | 514 |
Level 1 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 1 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 18 | 16 |
Level 1 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 690 | 498 |
Level 1 | Other long-term investment | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investment | 0 | 0 |
Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 2,307 | 2,266 |
Level 2 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 265 | 225 |
Level 2 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 76 | 148 |
Level 2 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 1,565 | 1,486 |
Level 2 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 73 | 60 |
Level 2 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 59 | 62 |
Level 2 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 269 | 285 |
Level 2 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 2 | Other long-term investment | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investment | 0 | 0 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets | 128 | 118 |
Level 3 | U.S. government and agencies | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | State and political subdivisions | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 16 | 17 |
Level 3 | Mortgage-backed/asset-backed securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Foreign government bonds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Available for sale debt securities | 0 | 0 |
Level 3 | Preferred securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 3 | Equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 0 | 0 |
Level 3 | Other long-term investment | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Other long-term investment | $ 112 | $ 101 |